check if member exists using enable_if

Question

Here\'s what I\'m trying to do:

template  struct Model
{
    vector vertices ;

    #if T has a .normal member
    void transform(         


        

Answer 1

This isn't an answer to your exact case, but it is an alternative answer to the question title and problem in general.

#include 
#include 

struct Foo {
    size_t length() { return 5; }
};

struct Bar {
    void length();
};

template length()), size_t>::value>
constexpr bool hasLengthHelper(int) { 
    return result;
}

template 
constexpr bool hasLengthHelper(...) { return false; }

template 
constexpr bool hasLength() {
    return hasLengthHelper(0);
}

// function is only valid if `.length()` is present, with return type `size_t`
template 
typename std::enable_if(), size_t>::type lengthOf (R r) {
  return r.length();
}

int main() {
    std::cout << 
      hasLength() << "; " <<
      hasLength>() << "; " <<
      hasLength() << ";" <<
      lengthOf(Foo()) <<
      std::endl;
    // 1; 0; 0; 5

    return 0;
}

Relevant https://ideone.com/utZqjk.

Credits to dyreshark on the freenode IRC #c++.